Because technical and ethical issues impose limitations on pathological studies of human subjects, disease models using animals known to be genetically very similar to human beings are valuable alternatives. Model animals are afflicted with a target disease, and various therapeutic agents are applied to search for treatment methods. However, because it is uncertain whether the same effects may be obtained in humans with use of therapeutic agents known to have therapeutic effects in animal disease models, and direct application of therapeutic agents identified in animal disease models for humans is prohibited, such therapeutic agents must undergo many procedures until they are approved for clinical application in humans.
Recently, efforts have been actively made to generate humanized animal models with a human-like immune system in order to establish animal disease models more effectively than ever. Among humanized animal models, in particular, methods of transplanting hematopoietic stem cells into immunodeficient mice have been established humanized mice. As a result of transplantation of human CD34+ cells into severe combined immune-deficient (SCID) mice, small numbers of human hematopoietic stem cell-derived cells were detected in all tissues and the cells were reconstituted in the tissues of the mice (Greiner D L, Hesselton R A and Shultz L D, Stem Cells 1998; 16: 166-77). When human CD34+ hematopoietic stem cells were transplanted into non-obese diabetic/SCID (NOD/SCID) mice with impaired NK cell function, human cells were greatly reconstituted in tissues than those of SCID mice. However, human cells were detected mainly in bone marrow, and human T cells were not still developed in the initial period after transplantation (Bente D A, Melkus M W, Gracia J V and Rico-Hesse R, J Virol 2005; 79:13797-9).
Although NOD/SCID mice lack T and B cells, the mice still have NK cell functions and take disadvantage of their short life spans due to spontaneous formation of lymphoma. In order to improve these limitations, NSG mice with impaired NK cell functions as well as defective T and B cells (NOD/SCID/IL2 receptor γ null) were developed (Shultz L D, et al., J. Immunol. 2005; 174: 6477-6489). Because NSG mice have life spans that are twice longer than those of NOD/SCID mice, they are appropriate for long-term monitoring after transplantation of human cells. It was confirmed that human B, T, and NK cells might be reconstituted by a single transplantation of hematopoietic stem cells without simultaneous transplantation of human hematopoietic tissues such as human fetal thymus or human fetal liver into NSG mice (Blood. 2002; 100: 3175-3182). When only hematopoietic stem cells were transplanted into NSG mice, 30 to 90% of human cells were detected due to successful engraftment and differentiation of the human cells. However, most of the B and T cells were not differentiated in normal ratios, and B cells were predominantly distributed unlike the distribution of immune cells in normal human peripheral blood.
Therefore, there still exists a need for a humanized mouse with a ratio of mature human B and T cells that is more similar to those in the human than those in previously established humanized mice.